1. Field of the Invention
The present invention relates to a wheel driving apparatus and an electric vehicle, and more specifically, to a wheel driving apparatus which drives a wheel with an electric motor, and to an electric vehicle including the wheel driving apparatus.
2. Description of the Related Art
Conventionally, electric vehicles such as electric scooters drive their wheels with an electric motor by controlling the motor's revolution (rotation speed) based on the amount of accelerator operation. When controlling the electric motor's rotation speed, a wide driving range from a high-torque low-rotation-speed to a low-torque high-rotation-speed is used so as to drive the electric motor efficiently.
As an electric motor to achieve this, there is known a variable air-gap permanent-magnet motor which is disclosed in JP-A 2005-168190, for example. The variable air-gap permanent-magnet motor includes a disc-shaped rotor provided with a permanent magnet, a stator having an armature coil and disposed to oppose the rotor, and a hydraulic mechanism for sliding thereby moving the stator inside the motor's casing. With this arrangement, by controlling the hydraulic mechanism, a gap length between the rotor and the stator is adjusted.
JP-A 2005-168190 discloses an arrangement in which a relationship between motor rotation speed and target gap length is stored in a reference map, for example, and when the motor is driven, the hydraulic mechanism is operated so as to achieve a target gap length which is appropriate to the actual motor rotation speed. The document also discloses an arrangement that a relationship between motor rotation speed and target gap length is stored in advance in a reference map, for example, in accordance with estimated motor shaft torque values, and when the motor is driven, a target gap length is obtained from the estimated shaft torque value and the motor rotation speed, and then the hydraulic mechanism is operated so as to achieve the obtained target gap length.
An output characteristic of electric motors differs from one motor to another depending on individual variability of the motor (for example, mechanical dimension tolerance of the air gap and magnetic flux density tolerance of the permanent magnets) as well as the motor's operating environment (for example, magnet temperature and power source voltage). For this reason, setting the target gap length in accordance with motor rotation speed as in the electric motor disclosed in JP-A 2005-168190 will result in an inability to achieve an appropriate gap length if the actual motor output characteristic is not identical with the expected characteristic (characteristic as designed).
Now, description will be made for the output characteristic of electric motors (a relationship between rotation speed and torque).
As shown in FIG. 26, electric motors have a characteristic that their maximum torque decreases linearly with an increase in the rotation speed (the amount of revolutions per unit time). Also, in order to protect motor driving circuits such as inverters, an upper limit current value is set for the electric current which is applied to the electric motor. Since the electric motor's torque is generally proportional to the applied current, the upper limit current value limits a maximum value of the applied current. For example, if the inverter's circuit element (FET) is set to have an upper limit current value of 100 A, a maximum torque will be a torque attained when this upper limit current value is applied. Therefore, actual drive control range for the electric motor is represented by a hatched range shown in FIG. 27.
When the gap length of the electric motor is changed, an output characteristic (rotation-speed vs. torque characteristic) of the electric motor changes. FIG. 28 shows an example where the gap length is changed in three levels. In FIG. 28, a solid line shows Characteristic 1 which is a characteristic when the gap length is short (gap length G1), a broken line shows Characteristic 3 which is a characteristic when the gap length is long (gap length G3), and a dashed line shows Characteristic 2 which is a characteristic when the gap length is in the middle (gap length G2: G1<G2<G3). Under the state of Characteristic 1, high torque is obtained but the motor cannot rotate at a high speed. Under the state of Characteristic 3, the motor can rotate at a high speed but high torque is not obtainable. Characteristic 2 is an intermediate characteristic between the two. Therefore, by changing the gap length, it is possible to achieve a drive control characteristic of the electric motor as indicated by a solid line in FIG. 29. For example, by changing the gap length in the pattern of G1→G2→G3, it is possible to change the output characteristic in the pattern of Characteristic 1→Characteristic 2→Characteristic 3. By using an electric motor which has such a capability of varying the gap length for a wheel drive motor of an electric vehicle, a vehicle characteristic capable of driving from a high-torque low-rotation-speed range through a low-torque high-rotation-speed range can be achieved.
In order to drive the electric motor efficiently, it is desirable that the gap length is switched at a switching timing where the electric motor is capable of performing at its maximum potential, i.e., on its output characteristic line. Here, reference will be made to FIG. 30 to explain the gap length switching timing. In this example, the gap length is varied in three levels (G1, G2, G3: G1<G2<G3). At a time of starting the electric motor, a short gap length G1 is selected in preference of torque over rotation speed. In this case, the state of the electric motor (torque, rotation speed) changes on Current Limiting Line L1 in a direction indicated by arrows. The current Limiting Line L1 shows torques obtained when the gap length is G1 and the applied current is the upper limit current value. Also, Current Limiting Line L2 shows torques obtained when the gap length is G2 and the applied current is the upper limit current value, whereas Current Limiting Line L3 shows torques which are obtained when the gap length is G3 and the applied current is the upper limit current value.
If the gap length is changed from G1 to G2 when the electric motor's rotation speed has increased to a cross point between Current Limiting Line L1 and Characteristic 1 line (rotation speed n1), it is possible to change the output characteristic at a point where the motor is performing at its maximum potential. At this point, the output torque decreases to a level limited by Current Limiting Line L2, and the rotation speed increases while maintaining this torque. Likewise, when the state of the electric motor (torque, rotation speed) has reached a cross point between Current Limiting Line L2 and Characteristic 2 line (rotation speed n2), the gap length is switched from G2 to G3. As described above, it is possible to make the electric motor perform at its maximum potential capacity by switching the gap length on the output characteristic line.
In a case where the switching of the gap length is based on motor rotation speed as in JP-A 2005-168190, there would be no problem in switching the gap length at those points where the rotation speed n1 or n2 has been detected if the output characteristic of electric motors is identical with the expected characteristic (characteristic as designed). However, if the actual output characteristic of the electric motor is different from the expected characteristic, the switching timing can be inappropriate and such problems may arise as the electric motor's potential is not utilized sufficiently or the drive feeling is not as good.
For example, take a case that the electric motor's torque tends to be weaker than the output characteristic shown in FIG. 30. Then, when the gap length is switched as shown in FIG. 31 at each point where the rotation speed n1 or n2 is detected, switching of the gap length tends to be too early as compared to the increase in rotation speed. In this case, the electric motor's potential capability cannot be utilized in the areas circled by broken lines. In other words, the rotation speed could have been increased further while maintaining the maximum torque, yet the gap length was switched, making it impossible to obtain advantageous torques available from the electric motor.
Conversely, in a case where the electric motor's torque tends to be stronger than the expected characteristics, switching timing will be too late if the gap length is switched as shown in FIG. 32, at each point where the rotation speed n1 or n2 is detected. In this case, the torque will drop dramatically before the gap length is switched as shown in the area enclosed by a broken line. This torque drop will result in a decreased drive feeling. For example, when the vehicle is starting to run, there will be a feeling for a moment that the vehicle is losing acceleration.